The physiological and anatomical analysis of conditioning would be facilitated by isolating and analyzing neuronal subsystems at which learned changes occur which are locally mediated. Description of the principles of plasticity there could be a key for understanding them in other brain regions. An auditory conditioned stimulus travels from the periphery, it is first processed in the central nervous system at the cochlear nucleus. Anatomical and physiological data suggest that a neural matrix complex enough to mediate a local plastic change may exist there, especially in the dorsal cochlear nucleus. Correlations between cochlear nucleus cell types and response characteristics will be done by staining neurons with intracellullr injections of the enzyme horseradish peroxidase after recording. Three cell groups, bushy cells in anterior ventral cochlear nucleus and the fusiform and giant cells in dorsal cochlear nucleus, will be studied extracellularly during and after conditioning. The nature of changed responses seen and when they appear in relation to behavioral learning will be described. The relation between position of a neuron in the tonotopic array and its probability of changing to a particular conditioner stimulus frequency will be examined. Neurons in primary auditory cortex will be studied with similar techniques. Data gathered and techniques developed in carrying out the proposed research will be used as a basis for future work in more central regions. The overall goal is to help understand auditory signal processing and to characterize learning centers anatomically and physiologically.